Optical recording medium, and optical information device

ABSTRACT

An optical recording medium and an optical information device that improve the quality of a servo signal and a reproduction signal. In the optical recording medium, when shape-wise thicknesses tr 1 , tr 2 , tr 3 , and tr 4  of a cover layer ( 42 ), a first intermediate layer ( 43 ), a second intermediate layer ( 44 ), and a third intermediate layer ( 45 ) are respectively converted into thicknesses t 1 , t 2 , t 3 , and t 4  of the respective corresponding layers each having a predetermined refractive index “no”, a defocus amount with respect to a layer having a refractive index nrα and a thickness trα (satisfying: 1≦α≦n (where α is a positive integer and n is an integer of 4 or more)), and a defocus amount with respect to a layer having the refractive index “no” and a thickness tα (satisfying: 1≦α≦n (where α is a positive integer and n is an integer of 4 or more)) are equal to each other. Further, the thicknesses t 1 , t 2 , t 3 , and t 4  satisfy |t 1 −(t 2 +t 3 +t 4 )|≧1 μm, a difference between any two values of the thicknesses t 1 , t 2 , t 3 , and t 4  is set to 1 μm or more, and |(t 1 +t 2 )−(t 3 +t 4 )|≧1 μm.

This application claims the benefit of U.S. Non-Provisional applicationSer. No. 12/269,289, filed Nov. 12, 2008.

TECHNICAL FIELD

The present invention relates to an optical recording medium forinformation recording or reproducing by irradiated light, and an opticalinformation device for recording or reproducing information with respectto the optical recording medium, and more particularly to an interlayerstructure of an optical recording medium having three or moreinformation recording surfaces.

BACKGROUND ART

There are known optical discs called as DVD or BD (Blu-ray disc), asexamples of the commercially available high-density and large-capacityoptical information recording media. In recent years, the optical discshave become widely used as recording media for recording images, music,and computer-readable data. There also has been proposed an optical dischaving plural recording layers, as disclosed in Patent literature 1 andPatent literature 2, to further increase the recording capacity.

FIG. 13 is a diagram showing an arrangement of a conventional opticalrecording medium and optical head device. An optical recording medium401 includes a first information recording surface 401 a closest to asurface 401 z of the optical recording medium 401, a second informationrecording surface 401 b second closest to the surface 401 z of theoptical recording medium 401, a third information recording surface 401c third closest to the surface 401 z of the optical recording medium401, and a fourth information recording surface 401 d farthest from thesurface 401 z of the optical recording medium 401.

A divergent beam 70 emitted from a light source 1 is transmitted througha collimator lens 53, and incident into a polarized beam splitter 52.The beam 70 incident into the polarized beam splitter 52 is transmittedthrough the polarized beam splitter 52, and converted into circularlypolarized light while being transmitted through a quarter wavelengthplate 54. Thereafter, the beam 70 is converted into a convergent beamthrough an objective lens 56, transmitted through a transparentsubstrate of the optical recording medium 401, and collected on one ofthe first information recording surface 401 a, the second informationrecording surface 401 b, the third information recording surface 401 c,and the fourth information recording surface 401 d formed in theinterior of the optical recording medium 401.

The objective lens 56 is so designed as to make a spherical aberrationzero at an intermediate depth position between the first informationrecording surface 401 a and the fourth information recording surface 401d. A spherical aberration corrector 93 shifts the position of thecollimator lens 53 in an optical axis direction. Thereby, sphericalaberration resulting from collecting light on the first through thefourth information recording surfaces 401 a through 401 d is removed.

An aperture 55 restricts the opening of the objective lens 56, and setsthe numerical aperture NA of the objective lens 56 to 0.85. The beam 70reflected on the fourth information recording surface 401 d istransmitted through the objective lens 56 and the quarter wavelengthplate 54, converted into linearly polarized light along an optical pathdisplaced by 90 degrees with respect to the outward path, and thenreflected on the polarized beam splitter 52. The beam 70 reflected onthe polarized beam splitter 52 is converted into convergent light whilebeing transmitted through a light collecting lens 59, and incident intoa photodetector 320 through a cylindrical lens 57. Astigmatism isimparted to the beam 70 while the beam 70 is transmitted through thecylindrical lens 57.

The photodetector 320 has unillustrated four light receiving sections.Each of the light receiving sections outputs a current signal dependingon a received light amount. A focus error (hereinafter, called as FE)signal by an astigmatism method, a tracking error (hereinafter, calledas TE) signal by a push-pull method, and an information (hereinaftercalled as RF) signal recorded in the optical recording medium 401 aregenerated, based on the current signals. The FE signal and the TE signalare amplified to an intended level, subjected to phase compensation, andthen supplied to actuators 91 and 92, whereby focus control and trackingcontrol are performed.

In this example, the following problem occurs, in the case where thethickness t1 between the surface 401 z of the optical recording medium401 and the first information recording surface 401 a, the thickness t2between the first information recording surface 401 a and the secondinformation recording surface 401 b, the thickness t3 between the secondinformation recording surface 401 b and the third information recordingsurface 401 c, and the thickness t4 between the third informationrecording surface 401 c and the fourth information recording surface 401d are equal to each other.

For instance, in the case where the beam 70 is collected on the fourthinformation recording surface 401 d to record or reproduce informationon or from the fourth information recording surface 401 d, a part of thebeam 70 is reflected on the third information recording surface 401 c.The distance from the third information recording surface 401 c to thefourth information recording surface 401 d, and the distance from thethird information recording surface 401 c to the second informationrecording surface 401 b are equal to each other. Accordingly, the partof the beam 70 reflected on the third information recording surface 401c forms an image on a backside of the second information recordingsurface 401 b, and reflected light from the backside of the secondinformation surface 401 b is reflected on the third informationrecording surface 401 c. As a result, the light reflected on the thirdinformation recording surface 401 c, the backside of the secondinformation recording surface 401 b, and the third information recordingsurface 401 c may be mixed with reflected light from the fourthinformation recording surface 401 d to be read.

Further, the distance from the second information recording surface 401b to the fourth information recording surface 401 d, and the distancefrom the second information recording surface 401 b to the surface 401 zof the optical recording medium 401 are equal to each other.Accordingly, a part of the beam 70 reflected on the second informationrecording surface 401 b forms an image on the backside of the surface401 z of the optical recording medium 401, and reflected light from thebackside of the surface 401 z is reflected on the second informationrecording surface 401 b. As a result, the light reflected on the secondinformation recording surface 401 b, the backside of the surface 401 z,and the second information recording surface 401 b may be mixed withreflected light from the fourth information recording surface 401 d tobe read.

As described above, there is a problem that reflected light from thefourth information recording surface 401 d to be read is superimposedand mixed with reflected light which forms an image on the backside ofthe other surface, with the result that informationrecording/reproducing is obstructed. Light containing reflected lightwhich forms an image on the backside of the other surface has a highcoherence, and forms a brightness/darkness distribution on a lightreceiving element by coherence. Since the brightness/darknessdistribution is varied depending on a change in phase difference withrespect to reflected light from the other surface, resulting from asmall thickness variation of an intermediate layer in an in-planedirection of an optical disc, the quality of a servo signal and areproduction signal may be considerably deteriorated. Hereinafter, theabove problem is called as a back focus problem in the specification.

In order to prevent the back focus problem, Patent literature 1discloses a method, wherein the interlayer distance between theinformation recording surfaces is gradually increased in the order fromthe surface 401 z of the optical recording medium 401 so that a part ofthe beam 70 may not form an image on the backside of the secondinformation recording surface 401 b and the backside of the surface 401z simultaneously when the beam 70 is collected on the fourth informationrecording surface 401 d to be read. The thicknesses t1 through t4 eachhas a production variation of ±10 μm. It is necessary to set thethicknesses t1 through t4 to different values from each other, also in acase where the thicknesses t1 through t4 are varied. In view of this, adifference in the thicknesses t1 through t4 is set to e.g. 20 μm. Inthis example, the thicknesses t1 through t4 are respectively set to 40μm, 60 μm, 80 μm, and 100 μm, and the total interlayer thicknesst(=t2+t3+t4) from the first information recording surface 401 a to thefourth information recording layer 401 d is set to 240 μm.

In the case where the thickness of a cover layer from the surface 401 zto the first information recording surface 401 a, and the thickness fromthe fourth information recording surface 401 d to the first informationrecording surface 401 a are equal to each other, light reflected on thefourth information recording surface 401 d is focused on the surface 401z, and reflected on the surface 401 z. The light reflected on thesurface 401 z is reflected on the fourth information recording surface401 d, and guided to the photodetector 320. A light flux which forms animage on the backside of the surface 401 z does not have informationrelating to pits or marks, unlike a light flux which forms an image onthe backside of the other information recording surface. However, in thecase where the number of information recording surfaces is large, thelight amount of light returning from the information recording surfacesis reduced, and the reflectance of the surface 401 z is relativelyincreased. As a result, coherence between a light flux reflected on thebackside of the surface 401 z, and a light flux reflected on a targetedinformation recording surface to be recorded or reproduced is generatedin the similar manner as in the case of a light flux reflected on thebackside of the other information recording surfaces, which mayconsiderably deteriorate the quality of a servo signal and areproduction signal.

In view of the above problem, Patent literature 2 proposes a distancebetween information recording layers (information recording surfaces) ofan optical disc. Patent literature 2 discloses the following structure.

An optical recording medium has four information recording surfaces,wherein the first through the fourth information recording surfaces aredefined in the order from a side closest to a surface of the opticalrecording medium. The distance from the medium surface to the firstinformation recording surface is set to 47 μm or less. The thicknessesof intermediate layers between the first through the fourth informationrecording surfaces are combination of a range from 11 to 15 μm, a rangefrom 16 to 21 μm, and a range of 22 μm or more. The distance from themedium surface to the fourth information recording surface is set to 100μm. The distance from the medium surface to the first informationrecording surface is set to 47 μm or less, and the distance from themedium surface to the fourth information recording surface is set to 100μm.

An optical disc system is adapted to detect light incident from a mediumsurface and reflected on an information recording surface. Accordingly,a refractive index of a transparent material constituting a transparentmember from the medium surface where light is transmitted to theinformation recording surface also affects the quality of a servo signaland a reproduction signal. However, there is no consideration anddescription about the refractive index in the disc structures disclosedin Patent literature 1 and Patent literature 2. Thus, both of thepublications do not consider an influence of a refractive index of atransparent material to the quality of a servo signal and a reproductionsignal.

CITATION LIST Patent Literature

-   Patent literature 1: JP 2001-155380A-   Patent literature 2: JP 2008-117513A

SUMMARY OF INVENTION

In view of the above, an object of the invention is to provide anoptical recording medium and an optical information device that enableto improve the quality of a servo signal and a reproduction signal.

An optical recording medium according to an aspect of the invention isan optical recording medium having a plurality of information recordingsurfaces. The optical recording medium includes: the first informationrecording surface closest to a medium surface of the optical recordingmedium where light is incident; the second information recording surfacesecond closest to the medium surface; the third information recordingsurface third closest to the medium surface; the fourth informationrecording surface fourth closest to the medium surface; a cover layerhaving a refractive index nr1 and formed between the medium surface andthe first information recording surface; a first intermediate layerhaving a refractive index nr2 and formed between the first informationrecording surface and the second information recording surface; a secondintermediate layer having a refractive index nr3 and formed between thesecond information recording surface and the third information recordingsurface; and a third intermediate layer having a refractive index nr4and formed between the third information recording surface and thefourth information recording surface, wherein in the case whereshape-wise thicknesses tr1, tr2, tr3, and tr4 of the cover layer, thefirst intermediate layer, the second intermediate layer, and the thirdintermediate layer are respectively converted into thicknesses t1, t2,t3, and t4 of the respective corresponding layers each having apredetermined refractive index “no”, a defocus amount with respect to alayer having a refractive index nrα and a thickness trα (satisfying:1≦α≦n (where α is a positive integer and n is an integer of 4 or more)),and a defocus amount with respect to a layer having the refractive index“no” and a thickness tα (satisfying: 1≦α≦n (where α is a positiveinteger and n is an integer of 4 or more)) are equal to each other, andthe thicknesses t1, t2, t3, and t4 satisfy |t1−(t2+t3+t4)|≧1 μm, adifference between any two values of the thicknesses t1, t2, t3, and t4is set to 1 μm or more in any case, and |(t1+t2)−(t3+t4)|≧1 μm.

In the above arrangement, in the case where shape-wise thicknesses tr1,tr2, tr3, and tr4 of the cover layer, the first intermediate layer, thesecond intermediate layer, and the third intermediate layer arerespectively converted into thicknesses t1, t2, t3, and t4 of therespective corresponding layers each having a predetermined refractiveindex “no”, a defocus amount with respect to a layer having a refractiveindex nrα and a thickness trα (satisfying: 1≦α≦n (where α is a positiveinteger and n is an integer of 4 or more)), and a defocus amount withrespect to a layer having the refractive index “no” and a thickness to(satisfying: 1≦α≦n (where α is a positive integer and n is an integer of4 or more)) are equal to each other, and the thicknesses t1, t2, t3, andt4 satisfy |t1−(t2+t3+t4)|≧1 μm, a difference between any two values ofthe thicknesses t1, t2, t3, and t4 is set to 1 μm or more in any case,and |(t1+t2)−(t3+t4)|≧1 μm.

According to the invention, the thicknesses t1, t2, t3, and t4 obtainedby converting the shape-wise thicknesses tr1, tr2, tr3, and tr4 of thecover layer, the first intermediate layer, the second intermediatelayer, and the third intermediate layer satisfy it |t1−(t2+t3+t4)|≧1 μm,a difference between any two values of the thicknesses t1, t2, t3, andt4 is set to 1 μm or more in any case, and |(t1+t2)−(t3+t4)|≧1 μm. Thisenables to prevent light from forming an image on the backside of thesurface of the optical recording medium, and suppress coherence betweenreflected light from the information recording surfaces to therebyimprove the quality of a servo signal and a reproduction signal.Further, since the distance between the surface of the optical recordingmedium and the information recording surface closest to the surface ofthe optical recording medium can be set to a large value, deteriorationof a reproduction signal in the case where there is a damage or a smearon the surface of the optical recording medium can be suppressed.

These and other objects, features and advantages of the presentinvention will become more apparent upon reading the following detaileddescription along with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a schematic arrangement of an opticalrecording medium embodying the invention, and an optical head device.

FIG. 2 is a diagram showing a layer structure of the optical recordingmedium in the embodiment of the invention.

FIG. 3 is a diagram showing reflected light from a fourth informationrecording surface, in the case where a beam is collected on the fourthinformation recording surface.

FIG. 4 is a diagram showing reflected light from a third informationrecording surface and a second information recording surface, in thecase where a beam is collected on the fourth information recordingsurface.

FIG. 5 is a diagram showing reflected light from the second informationrecording surface and a surface of the optical recording medium, in thecase where a beam is collected on the fourth information recordingsurface.

FIG. 6 is a diagram showing reflected light from the third informationrecording surface, a first information recording surface, and the secondinformation recording surface, in the case where a beam is collected onthe fourth information recording surface.

FIG. 7 is a diagram showing a relation between a difference ininterlayer thickness, and an amplitude of an FS signal.

FIG. 8 is a diagram showing a relation between an interlayer thicknessof an optical recording medium having information recording surfaces ofreflectances substantially equal to each other, and a jitter.

FIG. 9 is a diagram showing a layer structure of an optical recordingmedium as a modification of the embodiment of the invention.

FIG. 10 is an explanatory diagram showing a refractive index dependenceof a factor for converting a shape-wise thickness in terms of an actualrefractive index into a thickness in terms of a standard refractiveindex.

FIG. 11 is an explanatory diagram showing a refractive index dependenceof a factor for converting a thickness in terms of a standard refractiveindex into a shape-wise thickness in terms of an actual refractiveindex.

FIG. 12 is a diagram showing a schematic arrangement of an opticalinformation device embodying the invention.

FIG. 13 is a diagram showing an arrangement of a conventional opticalrecording medium and optical head device.

DESCRIPTION OF EMBODIMENTS

In the following, an embodiment of the invention is described referringto the accompanying drawings. The following embodiment is merely anexample embodying the invention, and does not limit the technical scopeof the invention.

Firstly, an optical recording medium embodying the invention isdescribed referring to FIGS. 1 and 2.

FIG. 1 is a diagram showing a schematic arrangement of an opticalrecording medium embodying the invention, and an optical head device.FIG. 2 is a diagram showing a layer structure of the optical recordingmedium in the embodiment. An optical head device 201 irradiates bluelaser light whose wavelength λ is 405 nm onto an optical recordingmedium 40 to reproduce a signal recorded in the optical recording medium40. Since the arrangement and the operation of the optical head device201 shown in FIG. 1 are substantially the same as the arrangement andthe operation of the optical head device shown in FIG. 13, detaileddescription thereof is omitted herein.

The optical recording medium 40 as an example has four informationrecording surfaces. As shown in FIG. 2, the optical recording medium 40has, in the order from a side closest to a surface 40 z of the opticalrecording medium 40, a first information recording surface 40 a, asecond information recording surface 40 b, a third information recordingsurface 40 c, and a fourth information recording surface 40 d.

The optical recording medium 40 is further provided with a cover layer42, a first intermediate layer 43, a second intermediate layer 44, and athird intermediate layer 45. The thickness t1 of the cover layer 42represents a thickness of a substrate from the surface 40 z to the firstinformation recording surface 40 a, the thickness t2 of the firstintermediate layer 43 represents a thickness of the substrate from thefirst information recording surface 40 a to the second informationrecording surface 40 b, the thickness t3 of the second intermediatelayer 44 represents a thickness of the substrate from the secondinformation recording surface 40 b to the third information recordingsurface 40 c, and the thickness t4 of the third intermediate layer 45represents a thickness of the substrate from the third informationrecording surface 40 c to the fourth information recording surface 40 d.

The distance d1 (≈t1) represents a distance from the surface 40 z to thefirst information recording surface 40 a, the distance d2 (≈t1+t2)represents a distance from the surface 40 z to the second informationrecording surface 40 b, the distance d3 (≈t1+t2+t3) represents adistance from the surface 40 z to the third information recordingsurface 40 c, and the distance d4 (≈t1+t2+t3+t4) represents a distancefrom the surface 40 z to the fourth information recording surface 40 d.

Now, problems to be solved in the case where an optical recording mediumhas four information recording surfaces are described. Coherence betweenreflected light from multiple surfaces is described referring to FIGS. 3through 7, as a first problem to be solved.

FIG. 3 is a diagram showing reflected light from the fourth informationrecording surface 40 d, in the case where a beam is collected on thefourth information recording surface 40 d. FIG. 4 is a diagram showingreflected light from the third information recording surface 40 c andthe second information recording surface 40 b, in the case where a beamis collected on the fourth information recording surface 40 d. FIG. 5 isa diagram showing reflected light from the second information recordingsurface 40 b and the surface 40 z, in the case where a beam is collectedon the fourth information recording surface 40 d. FIG. 6 is a diagramshowing reflected light from the third information recording surface 40c, the first information recording surface 40 a, and the secondinformation recording surface 40 b, in the case where a beam iscollected on the fourth information recording surface 40 d.

As shown in FIG. 3, a light flux collected on the fourth informationrecording surface 40 d for information reproducing or recording is splitinto the following light beams by semi-translucency of an informationrecording layer (an information recording surface).

Specifically, a light flux collected on the fourth information recordingsurface 40 d for information reproducing or recording is split into: abeam 70 shown in FIG. 3; a beam 71 (back focus light with respect to aninformation recording surface) shown in FIG. 4, a beam 72 (back focuslight with respect to a medium surface) shown in FIG. 5, and a beam 73shown in FIG. 6.

As shown in FIG. 3, the beam 70 is a beam reflected on the fourthinformation recording surface 40 d and emitted from the surface 40 z. Asshown in FIG. 4, the beam 71 is a beam reflected on the thirdinformation recording surface 40 c, focused and reflected on thebackside of the second information recording surface 40 b, reflected onthe third information recording surface 40 c, and emitted from thesurface 40 z. As shown in FIG. 5, the beam 72 is a beam reflected on thesecond information recording surface 40 b, focused and reflected on thebackside of the surface 40 z, reflected on the second informationrecording surface 40 b, and emitted from the surface 40 z. As shown inFIG. 6, the beam 73 is a beam which is not focused on the surface 40 zand the backsides of the information recording surfaces, but isreflected in the order of the third information recording surface 40 c,the backside of the first information recording surface 40 a, and thesecond information recording surface 40 b, and emitted from the surface40 z.

First, let us consider a case that the refractive indexes of the coverlayer 42, the first intermediate layer 43, the second intermediate layer44, and the third intermediate layer 45 are equal to each other. In thiscase, the refractive indexes of the respective corresponding layers areset to “no”.

For instance, in the case where the distance (thickness t4) between thefourth information recording surface 40 d and the third informationrecording surface 40 c, and the distance (thickness t3) between thethird information recording surface 40 c and the second informationrecording surface 40 b are equal to each other, the beam 70 and the beam71 pass a common optical path when exiting from the surface 40 z.Accordingly, the beam 70 and the beam 71 are incident into aphotodetector 320 with an identical light flux diameter. Similarly, inthe case where the distance (thickness t4+thickness t3) between thefourth information recording surface 40 d and the second informationrecording surface 40 b, and the distance (thickness t2+thickness t1)between the second information recording surface 40 b and the surface 40z are equal to each other, the beam 70 and the beam 72 pass a commonoptical path when exiting from the surface 40 z. Accordingly, the beam70 and the beam 72 are incident into the photodetector 320 with anidentical light flux diameter. In the case where the distance (thicknesst2) between the second information recording surface 40 b and the firstinformation recording surface 40 a, and the distance (thickness t4)between the fourth information recording surface 40 d and the thirdinformation recording surface 40 c are equal to each other, the beam 70and the beam 73 pass a common optical path when exiting from the surface40 z. Accordingly, the beam 70 and the beam 73 are incident into thephotodetector 320 with an identical light flux diameter.

The light intensities of the beams 71 through 73 as reflected light frommultiple surfaces are small, as compared with the light intensity of thebeam 70. However, coherent contrast does not depend on a light intensitybut depends on a light intensity ratio of light amplitude, and the lightamplitude is a square root of the light intensity. Accordingly, even asmall difference between light intensities results in a large coherentcontrast. In the case where the beams 70 through 73 are incident intothe photodetector 320 with an identical light flux diameter, aninfluence by coherence between the beams is large. Further, a lightreceiving amount by the photodetector 320 is greatly varied, resultingfrom a small change in thickness between the information recordingsurfaces, which makes it difficult to stably detect a signal.

FIG. 7 is a diagram showing a relation between a difference ininterlayer thickness, and an amplitude of an FS signal. FIG. 7 shows anamplitude of an FS signal (a sum of light intensities) with respect to adifference in interlayer thickness, in the case where the lightintensity ratio between the beam 70; and the beam 71, the beam 72, orthe beam 73 is set to 100:1, and the refractive indexes of the coverlayer 42 and the first intermediate layer 43 are each set to about 1.60(1.57). Referring to FIG. 7, the axis of abscissas indicates adifference in interlayer thickness, and the axis of ordinate indicatesan amplitude of an FS signal. The FS signal amplitude is a valueobtained by normalizing light solely composed of the beam 70 to bedetected by the photodetector 320 by a DC light amount, assuming thatthere is no reflection from the other information recording surfaces. Inthis embodiment, an interlayer means a layer between a surface of theoptical recording medium and an information recording surface, and alayer between information recording surfaces adjacent to each other. Asshown in FIG. 7, it is obvious that the FS signal is sharply changedwhen the difference in interlayer thickness becomes about 1 μm or less.

Similarly to the beam 72 shown in FIG. 5, in the case where thedifference between the thickness t1 of the cover layer 42, and the sum(t2+t3+t4) of the thicknesses of the first through the thirdintermediate layers 43 through 45 is 1 μm or less, a problem such asvariation of the FS signal also occurs.

As a second problem to be solved, an exceedingly small interlayerdistance between adjacent information recording surfaces causes aninfluence of crosstalk from the adjacent information recording surface.In view of this, an interlayer distance of a predetermined value or moreis necessary. Accordingly, various interlayer thicknesses areinvestigated, and an interlayer thickness which minimizes the influenceis determined.

FIG. 8 is a diagram showing a relation between an interlayer thicknessof an optical recording medium having information recording surfaces ofreflectances substantially equal to each other, and a jitter. Therefractive index of the intermediate layer is set to about 1.60.Referring to FIG. 8, the axis of abscissas indicates an interlayerthickness, and the axis of ordinate indicates a jitter value. As theinterlayer thickness is reduced, the jitter is deteriorated. Theinterlayer thickness where the jitter starts increasing is about 10 μm,and in the case where the interlayer thickness becomes 10 μm or less,the jitter is seriously deteriorated. Therefore, an optimum minimumvalue of the interlayer thickness is 10 μm.

Referring to FIG. 2, an arrangement of the optical recording medium 40in the embodiment of the invention is described. In the embodiment, thestructure of a four-layer disc (the optical recording medium 40) isdefined in such a manner as to secure the following conditions (1)through (3) in order to eliminate an adverse effect of reflected lightfrom the other information recording surfaces or a disc surface,considering a thickness variation among products.

Condition (1): The difference between the thickness t1 of the coverlayer 42, and the sum (t2+t3+t4) of the thicknesses t2 through t4 of thefirst through the third intermediate layers 43 through 45 is set to 1 μmor more. In other words, the thicknesses t1, t2, t3, and t4 satisfy|t1−(t2+t3+t4)|≧1 μm.

Condition (2): The difference between any two values of the thicknessest1, t2, t3, and t4 is set to 1 μm or more in any case.

Condition (3): The difference between the sum (t1+t2) of the thicknesst1 of the cover layer 42 and the thickness t2 of the first intermediatelayer 43, and the sum (t3+t4) of the thickness t3 of the secondintermediate layer 44 and the thickness t4 of the third intermediatelayer 45 is set to 1 μm or more. In other words, the thicknesses t1, t2,t3, and t4 satisfy |t1−(t2+t3+t4)|≧1 μm.

There are other combinations of interlayer thicknesses. However, in thecase where the thickness t1 of the cover layer is set to a valueapproximate to the sum (t2+t3+t4) of the thicknesses t2 through t4 ofthe first through the third intermediate layers 43 through 45, there isno need of considering the other combinations. Therefore, description onthe other combinations is omitted herein.

FIG. 9 is a diagram showing a layer structure of an optical recordingmedium as a modification of the embodiment of the invention. An opticalrecording medium 30 shown in FIG. 9 has three information recordingsurfaces. As shown in FIG. 9, the optical recording medium 30 has, inthe order from a side closest to a surface 30 z of the optical recordingmedium 30, a first information recording surface 30 a, a secondinformation recording surface 30 b, and a third information recordingsurface 30 c. The optical recording medium 30 is further provided with acover layer 32, a first intermediate layer 33, and a second intermediatelayer 34.

The thickness t1 of the cover layer 32 represents a thickness of asubstrate from the surface 30 z to the first information recordingsurface 30 a, the thickness t2 of the first intermediate layer 33represents a thickness of the substrate from the first informationrecording surface 30 a to the second information recording surface 30 b,and the thickness t3 of the second intermediate layer 34 represents athickness of the substrate from the second information recording surface30 b to the third information recording surface 30 c.

The distance d1 (≈t1) represents a distance from the surface 30 z to thefirst information recording surface 30 a, the distance d2 (≈t1+t2)represents a distance from the surface 30 z to the second informationrecording surface 30 b, and the distance d3 (≈t1+t2+t3) represents adistance from the surface 30 z to the third information recordingsurface 30 c.

In the foregoing description, the structure of the four-layer disc isconcretely described. In the case where a three-layer disc as shown inFIG. 9 is produced, the structure of the three-layer disc (the opticalrecording medium 30) is defined in such a manner as to secure thefollowing conditions (1) and (2).

Condition (1): The difference between the thickness t1 of the coverlayer 32, and the sum (t2+t3) of the thicknesses t2 and t3 of the firstintermediate layer 33 and the second intermediate layer 34 is set to 1μm or more. In other words, the optical recording medium 30 satisfies|t1−(t2+t3)|≧1 μm.

Condition (2): The difference between any two values of the thicknessest1, t2, and t3 is set to 1 μm or more in any case.

Concerning a (N−1)-layer disc (where n is a positive integer equal to ormore than 4), the above condition generally means that a differencebetween the sum of the thickness “ti” through the thickness “tj”, andthe sum of the thickness “tk” through the thickness “tm” is necessarilyset to 1 μm or more, assuming that t1 is a thickness of the cover layer,and t2 through tN are thicknesses of the first through the N-thintermediate layers, where i, j, k, and m are each an arbitrary positiveinteger, and i≦j<k≦m≦N. The cover layer thickness corresponds to adistance from the surface of the optical recording medium to theinformation recording surface closest to the medium surface. The abovedescription is applied to the description that a distance from thesurface of the optical recording medium to the information recordingsurface second closest to the medium surface is defined as d2, adistance from the surface of the optical recording medium to theinformation recording surface third closest to the medium surface isdefined as d3, and a distance from the surface of the optical recordingmedium to the information recording surface fourth closest to the mediumsurface is defined as d4 in the same manner as described above.

Further, all the intermediate layer thicknesses are each set to 10 μm ormore to solve the second problem.

The foregoing description has been made based on the premise that therefractive indexes of the cover layer and the intermediate layers areequal to the standard value, and all the refractive indexes of the coverlayer and the intermediate layers are equal to each other. In thefollowing, described is a case that the refractive indexes of the coverlayer and the intermediate layers are different from the standard value,or the refractive indexes of the cover layer and the intermediate layersare different from each other among the layers.

The back focus problem as the first problem occurs because the size andthe shape are similar to each other between signal light, and reflectedlight from the other information recording surface on the photodetector320. In the case where the refractive index is set to about 1.60, it ispossible to avoid the back focus problem, as far as a difference betweenthe focus position of signal light, and the focus position of reflectedlight from the other information recording surface is smaller than 1 μmthe optical axis direction on the side of the optical recording medium.When the refractive index is set to about 1.60, crosstalk resulting froman adjacent information recording surface, as the second problem, occursin the case where a defocus amount of signal light is smaller than 10 μmon an adjacent track.

In both of the cases, a defocus amount is an important factor to beconsidered. The defocus amount corresponds to the size of reflectedlight from the other information recording surface, or the size of avirtual image of reflected light from the other information recordingsurface at a position where signal light is focused. Let it be assumedthat the radius of reflected light from the other information recordingsurface, or the radius of a virtual image of reflected light from theother information recording surface is RD. Since reflected light fromthe other information recording surface whose radius is RD is projectedonto the photodetector 320, coherence and the magnitude of crosstalkdepend on the size of the reflected light. The size of the reflectedlight may be defined as a divergent amount of light resulting from aninterlayer thickness. We found that in order to avoid the back focusproblem and the crosstalk problem in the case where the refractive indexis set to a value other than 1.60, it is necessary to define a conditionthat makes a defocus amount i.e. the size of reflected light from theother information recording surface or the size of a virtual image ofreflected light from the other information recording surfacesubstantially equal to each other. The above technique may be defined asa technique of converting an interlayer thickness, referring to adivergent amount of light resulting from an interlayer thickness.

A condition that makes a defocus (the size of reflected light from theother information recording surface, or the size of a virtual image ofreflected light from the other information recording surface) withrespect to a layer having a refractive index “nr” different from astandard refractive index “no” and a shape-wise thickness “dr” equal toa defocus with respect to a layer having the standard refractive index“no” and a shape-wise thickness “do” is expressed by the followingequations (1) and (2).NA=nr·sin(θr)=no·sin(θo)  (1)RD=dr·tan(θr)=do·tan(θo)  (2)

In the equations, NA represents a numerical aperture of an objectivelens 56 for converging light onto an optical recording medium. Forinstance, NA=0.85. The symbols θr and θo respectively representconvergence angles of light in materials having the refractive index“nr” and the refractive index “no”. The symbol RD represents a radius ofreflected light from the other information recording surface, or aradius of a virtual image of reflected light from the other informationrecording surface. The symbols “sin” and “tan” respectively represent asine function and a tangent function. The standard refractive index “no”is set to e.g. 1.60, and more preferably set to 1.57.

The convergence angle θr is expressed by the following equation (3), andthe convergence angle θo is expressed by the following equation (4),based on the equation (1).θr=arcsin(NA/nr)  (3)θo=arcsin(NA/no)  (4)

In the equations, arcsin represents an inverse sine function.

The thickness “do” is expressed by the following equation (5), and thethickness “dr” is expressed by the following equation (6), based on theequation (2).do=dr·tan(θr)/tan(θo)  (5)dr=do·tan(θo)/tan(θr)  (6)

The thickness “do” is calculated using the equation (5) to derive thethickness of a layer having the refractive index “no” with respect tothe shape-wise thickness “dr” of a layer having the refractive index“nr”.

Conversely, the thickness “dr” is calculated using the equation (6) toderive the shape-wise thickness “dr” of a layer having the refractiveindex “nr” with respect to the thickness “do” of a layer having therefractive index “no”.

The factor portion in the equation (5) i.e. tan(θr)/tan(θo) is expressedas a function of the refractive index “nr” in FIG. 10. The factorportion in the equation (6) i.e. tan(θo)/tan(θr) is expressed as afunction of the refractive index “nr” in FIG. 11.

FIG. 10 is an explanatory diagram showing a refractive index dependenceof a factor for converting a shape-wise thickness in terms of an actualrefractive index into a thickness in terms of a standard refractiveindex. FIG. 11 is an explanatory diagram showing a refractive indexdependence of a factor for converting a thickness in terms of a standardrefractive index into a shape-wise thickness in terms of an actualrefractive index.

As an example, there is described a relation between the thickness t1 ofthe cover layer, and the sum of the thicknesses t2 through t4 of thefirst through the third intermediate layers of the four-layer disc (theoptical recording medium 40). Let us consider a case that all therefractive indexes of the layers are set to a standard refractive index“no” i.e. set to 1.60, the thickness t1 of the cover layer is set to 54μm, the thickness t2 of the first intermediate layer is set to 10 μm,the thickness t3 of the second intermediate layer is set to 21 μm, andthe thickness t4 of the third intermediate layer is set to 19 μm. Thesum of the thickness t2 of the first intermediate layer through thethickness t4 of the third intermediate layer becomes 50 μm. In thiscase, the difference between the thickness t1 of the cover layer, andthe sum of the thicknesses t2 through t4 of the first through the thirdintermediate layers is 4 μm, which is significantly larger than 1 μm.

If, however, the refractive index “nr” of the cover layer is set to1.70, a different result is obtained, even if the shape-wise thicknesstr1 of the cover layer remains the same i.e. set to 54 μm. It isobvious, from the equations (3) and (5) or from FIG. 10, that thethickness tr1 of a layer having the refractive index “nr” is convertedinto the thickness t1 of a layer having the standard refractive index“no” by multiplying the thickness tr1 by 0.921. As a result, thethickness t1 of the layer having the refractive index “no” is set to:t1=0.921×tr1=49.7 μm, which is smaller than 50 μm i.e. the sum of thethicknesses t2 through t4 of the first through the third intermediatelayers.

Conversely, it is obvious from the equations (4) and (6) or from FIG. 11that a difference between the thickness tr1 of the cover layer, and thesum of the thicknesses t2 through t4 of the first through the thirdintermediate layers is set to 1 μm or more, and the thickness tr1 of thecover layer is set to 51 μm or more by multiplying the thickness t1 of alayer having the refractive index “no” by 1.086. In other words, thethickness tr1 of the layer having the refractive index “nr” is set to:tr1=51×1.086≈55.4 μm. Accordingly, it is necessary to set the shape-wisethickness tr1 of the cover layer to 55.4 μm or more, in the case wherethe refractive index “nr” is set to 1.70. The above example is merely anexample, and the invention may embrace a value parameter other than theabove.

It is also necessary to satisfy a specific condition about the thicknessof the cover layer and the thicknesses of the intermediate layers fromanother aspect. It is desirable to set the cover layer thickness and theintermediate layer thicknesses in a predetermined range including astandard value to perform a stable focus jumping operation. A focusjumping operation is an operation of changing a focus position from acertain information recording surface to another information recordingsurface. In performing a focus jumping operation, it is desirable tosecure a focus error signal of good quality with respect to a targetedinformation recording surface by e.g. moving a collimator lens 53 priorto a focus jumping operation to stably obtain a focus error signal withrespect to the targeted information recording surface. In view of this,it is desirable to set a difference in spherical aberration betweeninformation recording surfaces in a predetermined range.

If the refractive index is changed, the spherical aberration amount ischanged, even if the thickness is unchanged. Accordingly, it isdesirable to set a targeted value or an allowable range of anintermediate layer thickness in such a manner that the sphericalaberration amount falls in a predetermined range.

Further, in the case where the refractive index of a predetermined layer(the cover layer or the intermediate layer) is nr(min)≦nr≦nr(max), inthe aspect of avoiding the back focus problem and the crosstalk problem,the thickness “dr” of a layer having the refractive index “nr” isconverted into a thickness corresponding to the thickness of theintermediate layer having the standard refractive index “no” byimplementing the equations: θr(min)=arcsin(NA/nr(min)) andθr(max)=arcsin(NA/nr(max)), and using the equation:do=dr·tan(θr)/tan(θo) in the similar manner as described above; andjudgment is made as to whether the obtained thickness is proper.

The optical recording medium in the embodiment is not limited to one ofa rewritable disc, a recordable disc, and a read only disc, but may beany of these discs.

Signal fluctuation and signal quality deterioration resulting from theback focus problem occur, in the case where the sizes or the shapes arethe same with each other between signal light, and reflected light fromthe other information recording surface on the photodetector. A statethat the sizes or the shapes are the same with each other between signallight, and reflected light from the other information recording surfaceon the photodetector means a state that focus positions appear to be thesame with each other between signal light, and reflected light from theother information recording surface, including a virtual image ofreflected light from the other information recording surface. Opticalpaths of signal light and reflected light from the other informationrecording surface are partly different from each other on an opticalpath in a transparent substrate of an optical disc. In the case wheredefocus amounts resulting from a difference in optical path are equal toeach other, the focus position of signal light, and the focus positionof reflected light from the other information recording surface appearto be the same with each other. In the case where divergences ofconvergent light i.e. the radii of convergent light are the same witheach other between signal light, and reflected light from the otherinformation recording surface, it is judged that defocus amountsresulting from a substrate thickness are equal to each other.

In view of the above, calculation based on the divergent radius R of alight spot resulting from a substrate thickness is made in order tojudge whether the back focus problem can be avoided by setting theshape-wise thickness “dr” in terms of the refractive index “nr”. In thisexample, the shape-wise thickness indicates a material thickness, andmay also be called as a physical thickness.

An interlayer coherence resulting from a reduced intermediate layerthickness can be avoided, if the spot configuration (the radius R) on anadjacent layer is sufficiently large. In view of this, calculation basedon the divergent radius R of a light spot resulting from a substratethickness is made in order to judge whether the interlayer coherence canbe avoided by setting the shape-wise thickness “dr” of a layer havingthe refractive index “nr”.

Assuming that the thickness of a cover layer or an intermediate layer is“t”, the numerical aperture of a light spot is NA (NA=0.85), and theconvergence angle of light in a substrate is θ, since NA=n·sin(θ),θ=arcsin(NA/n). In this equation, “arcsin” represents an inverse sinefunction. The divergent radius R of a light spot can be calculated byR=t·tan(θ).

The standard refractive index is defined as “no”, the thickness of alayer having the standard refractive index “no” is defined as “to”, andthe convergence angle of light in a substrate of the layer is defined as“θo”. The standard refractive index “no” is set to e.g. 1.60. The layer(targeted layer) constituting a thickness portion of a transparentsubstrate of an actual optical disc is indicated with the suffix “r”,the refractive index of the targeted layer is defined as “nr”, theshape-wise thickness of the targeted layer is defined as “tr”, and theconvergence angle of light in a substrate of the targeted layer isdefined as “θr”. In this case, the convergence angles θo and θr arerespectively expressed by: θo=arcsin(NA/no) and θr=arcsin(NA/nr).

The divergent radius R of a light spot is expressed by:R=tr·tan(θr)=to·tan(θo). Accordingly, the thickness “to” of a layerhaving the standard refractive index “no” is expressed by:to=tr·tan(θr)/tan(θo))=tr·f(nr).

The function f(nr) is a factor for deriving the thickness “to” of alayer having the standard refractive index “no” with respect to theshape-wise thickness “tr”, and is the function shown in the graph ofFIG. 10.

For instance, let us consider a four-layer disc having four layers ofinformation recording surfaces. The four-layer disc (the opticalrecording medium 40) has, in the order from the surface (a lightincident surface) 40 z of the disc, the first information recordingsurface 40 a, the second information recording surface 40 b, the thirdinformation recording surface 40 c, and the fourth information recordingsurface 40 d. The four-layer disc is further provided with the coverlayer 42 between the light incident surface 40 z and the firstinformation recording surface 40 a, the first intermediate layer 43between the first information recording surface 40 a and the secondinformation recording surface 40 b, the second intermediate layer 44between the second information recording surface 40 b and the thirdinformation recording surface 40 c, and the third intermediate layer 45between the third information recording surface 40 c and the fourthinformation recording surface 40 d.

Let it be assumed that the shape-wise thickness of the cover layer 42 istr1, and the actual refractive index of the cover layer 42 is nr1; theshape-wise thickness of the first intermediate layer 43 is tr2, and theactual refractive index of the first intermediate layer 43 is nr2; theshape-wise thickness of the second intermediate layer 44 is tr3, and theactual refractive index of the second intermediate layer 44 is nr3; andthe shape-wise thickness of the third intermediate layer 45 is tr4, andthe actual refractive index of the third intermediate layer 45 is nr4.

Converting the thicknesses tr1, tr2, tr3, and tr4 of the cover layer 42and the first through the third intermediate layers 43 through 45respectively into the thicknesses t1, t2, t3, and t4 of the cover layer42 and the first through the third intermediate layers 43 through 45each having the standard refractive index “no”, based on a defocusamount, yields a result: t1=tr1×f(nr1), t2=tr2×f(nr2), t3=tr3×f(nr3),and t4=tr4×f(nr4).

Normally, the thickness of the cover layer is larger than the thicknessof the intermediate layer. In view of this, the four-layer disc shouldsatisfy all the conditions: |t1−(t2+t3+t4)|≧1 μm, |t2−t3|≧1 μm,|t3−t4)|≧1 μm, and |t2−t4|≧1 μm to avoid the back focus problem.

Further, the four-layer disc should satisfy all the conditions: t2≧10μm, t3≧10 μm, and t4≧10 μm to avoid the interlayer coherence. In otherwords, the thicknesses t1, t2, t3, and t4 of the cover layer 42, thefirst intermediate layer 43, the second intermediate layer 44, and thethird intermediate layer 45 are each set to 10 μm or more.

As described above, the optical recording medium 40 includes the firstinformation recording surface 40 a closest to the light incident surface40 z of the optical recording medium 40, the second informationrecording surface 40 b second closest to the surface 40 z, the thirdinformation recording surface 40 c third closest to the surface 40 z,the fourth information recording surface 40 d fourth closest to thesurface 40 z, the cover layer 42 having the refractive index nr1 andformed between the surface 40 z and the first information recordingsurface 40 a, the first intermediate layer 43 having the refractiveindex nr2 and formed between the first information recording surface 40a and the second information recording surface 40 b, the secondintermediate layer 44 having the refractive index nr3 and formed betweenthe second information recording surface 40 b and the third informationrecording surface 40 c, and the third intermediate layer 45 having therefractive index nr4 and formed between the third information recordingsurface 40 c and the fourth information recording surface 40 d.

Further, in the case where the shape-wise thicknesses tr1, tr2, tr3, andtr4 of the cover layer 42, the first intermediate layer 43, the secondintermediate layer 44, and the third intermediate layer 45 arerespectively converted into the thicknesses t1, t2, t3, and t4 of therespective corresponding layers each having the predetermined refractiveindex “no”, the defocus amount with respect to a layer having therefractive index nrα and the thickness trα (satisfying: 1≦α≦n (where αis a positive integer and n is an integer of 4 or more)) is equal to thedefocus amount with respect to a layer having the refractive index “no”and the thickness tα (satisfying: 1≦α≦n (where α is a positive integerand n is an integer of 4 or more)); and the thicknesses t1, t2, t3, andt4 satisfy |t1−(t2+t3+t4)|≦1 μm, a difference between any two values ofthe thicknesses t1, t2, t3, and t4 is set to 1 μm or more in any case,and |(t1+t2)−(t3+t4)|≦1 μm.

Thus, the thicknesses t1, t2, t3, and t4 obtained by converting theshape-wise thickness tr1, tr2, tr3, and tr4 of the cover layer 42, thefirst intermediate layer 43, the second intermediate layer 44, and thethird intermediate layer 45 satisfy |t1−(t2+t3+t4)|≦1 μm, a differencebetween any two values of the thicknesses t1, t2, t3, and t4 is set to 1μm or more in any case, and |(t1+t2)−(t3+t4)|≦1 μm. This enables toprevent light from forming an image on the backside of the surface ofthe optical recording medium, and suppress coherence between reflectedlight from the information recording surfaces to thereby improve thequality of a servo signal and a reproduction signal.

Further, since the distance between the surface of the optical recordingmedium and the information recording surface closest to the surface ofthe optical recording medium can be set to a large value, deteriorationof a reproduction signal in the case where there is a damage or a smearon the surface of the optical recording medium can be suppressed.

Further, assuming that the shape-wise thicknesses tr1, tr2, tr3, and tr4of the cover layer 42, the first intermediate layer 43, the secondintermediate layer 44, and the third intermediate layer 45 arerespectively converted into the thicknesses t1, t2, t3, and t4 of therespective corresponding layers each having the predetermined refractiveindex “no”, the thickness of a layer having the refractive index nrα isset to trα (satisfying: 1≦α≦n (where α is a positive integer and n is aninteger of 4 or more)), the convergence angle of light in the layerhaving the refractive index nrα is set to θrα (satisfying: 1≦α≦n (whereα is a positive integer and n is an integer of 4 or more)), thethickness of a layer having the refractive index “no” is set to tα(satisfying: 1≦α≦n (where α is a positive integer and n is an integer of4 or more)), and the convergence angle of light in the layer having therefractive index “no” is set to θo, if the thickness trα is convertedinto the thickness tα based on the following equation, the thicknessest1, t2, t3, and t4 satisfy |t1−(t2+t3+t4)≦1 μm, a difference between anytwo values of the thicknesses t1, t2, t3, and t4 is set to 1 μm or morein any case, and |(t1+t2)−(t3+t4)|≦1 μm.tα=trα·(tan(θrα)/tan(θo))

In the case where the range of the thickness tα of a layer having therefractive index “no” and whose spherical aberration amount falls in apredetermined allowable range is converted into a range of the thicknesstrα of a layer having the refractive index nrα, preferably, thethickness trα may be included in the range of the thickness trα afterconversion.

As another example, let us consider a case that a three-layer dischaving three recording layers is produced. A three-layer disc (theoptical recording medium 30) has, in the order from the surface (a lightincident surface) 30 z of the disc, the first information recordingsurface 30 a, the second information recording surface 30 b, and thethird information recording surface 30 c. The three-layer disc isfurther provided with the cover layer 32 between the light incidentsurface 30 z and the first information recording surface 30 a, the firstintermediate layer 33 between the first information recording surface 30a and the second information recording surface 30 b, and the secondintermediate layer 34 between the second information recording surface30 b and the third information recording surface 30 c.

Let it be assumed that the shape-wise thickness of the cover layer 32 istr1, and the actual refractive index of the cover layer 32 is nr1; theshape-wise thickness of the first intermediate layer 33 is tr2, and theactual refractive index of the first intermediate layer 33 is nr2; andthe shape-wise thickness of the second intermediate layer 34 is tr3, andthe actual refractive index of the second intermediate layer 34 is nr3.

Converting the thicknesses tr1, tr2, and tr3 of the cover layer 32, thefirst intermediate layer 33, and the second intermediate layer 34respectively into the thicknesses t1, t2, and t3 of the cover layer 32,the first intermediate layer 33, and the second intermediate layer 34each having the standard refractive index “no”, based on a defocusamount, yields a result: t1=tr1×f(nr1), t2=tr2×f(nr2), andt3=tr3×f(nr3).

Normally, the thickness of the cover layer is larger than the thicknessof the intermediate layer. In view of this, the three-layer disc shouldsatisfy all the conditions: |t1−(t2−t3)|≦1 μm, and |t2−t3|≦1 μm to avoidthe back focus problem.

Further, the three-layer disc should satisfy all the conditions: t2≦10μm, and t3≦10 μm to avoid the interlayer coherence. In other words, thethicknesses t1, t2, and t3 of the cover layer 32, the first intermediatelayer 33, and the second intermediate layer 34 are each set to 10 μm ormore.

As described above, the optical recording medium 30 includes the firstinformation recording surface 30 a closest to the light incident surface30 z of the optical recording medium 30, the second informationrecording surface 30 b second closest to the surface 30 z, the thirdinformation recording surface 30 c third closest to the surface 30 z,the cover layer 32 having the refractive index nr1 and formed betweenthe surface 30 z and the first information recording surface 30 a, thefirst intermediate layer 33 having the refractive index nr2 and formedbetween the first information recording surface 30 a and the secondinformation recording surface 30 b, and the second intermediate layer 34having the refractive index nr3 and formed between the secondinformation recording surface 30 b and the third information recordingsurface 30 c.

Further, in the case where the shape-wise thicknesses tr1, tr2, and tr3of the cover layer 32, the first intermediate layer 33, and the secondintermediate layer 34 are respectively converted into the thicknessest1, t2, and t3 of the respective corresponding layers each having thepredetermined refractive index “no”, the defocus amount with respect toa layer having the refractive index nrα and the thickness trα(satisfying: 1≦α≦n (where α is a positive integer and n is an integer of4 or more)) is equal to the defocus amount with respect to a layerhaving the refractive index “no” and the thickness tα (satisfying: 1≦α≦n(where α is a positive integer and n is an integer of 4 or more)); andthe thicknesses t1, t2, and t3 satisfy |t1−(t2+t3)|≧1 μm, and adifference between any two values of the thicknesses t1, t2, and t3 isset to 1 μm or more in any case.

Thus, the thicknesses t1, t2, and t3 obtained by converting theshape-wise thickness tr1, tr2, and tr3 of the cover layer 32, the firstintermediate layer 33, and the second intermediate layer 34 satisfy|t1−(t2+t3)|≧1 μm, and a difference between any two values of thethicknesses t1, t2, and t3 is set to 1 μm or more in any case. Thisenables to prevent light from forming an image on the backside of thesurface of the optical recording medium, and suppress coherence betweenreflected light from the information recording surfaces to therebyimprove the quality of a servo signal and a reproduction signal.

Further, since the distance between the surface of the optical recordingmedium and the information recording surface closest to the surface ofthe optical recording medium can be set to a large value, deteriorationof a reproduction signal in the case where there is a damage or a smearon the surface of the optical recording medium can be suppressed.

Further, assuming that the shape-wise thicknesses tr1, tr2, and tr3 ofthe cover layer 32, the first intermediate layer 33, and the secondintermediate layer 34 are respectively converted into the thicknessest1, t2, and t3 of the respective corresponding layers each having thepredetermined refractive index “no”, the thickness of a layer having therefractive index nrα is set to trα (satisfying: 1≦α≦n (where α is apositive integer and n is an integer of 3 or more)), the convergenceangle of light in the layer having the refractive index nrα is set toθrα (satisfying: 1≦α≦n (where α is a positive integer and n is aninteger of 3 or more)); the thickness of a layer having the refractiveindex “no” is set to tα (satisfying: 1≦α≦n (where α is a positiveinteger and n is an integer of 3 or more)), and the convergence angle oflight in the layer having the refractive index “no” is set to θo, if thethickness trα is converted into the thickness tα based on the followingequation, the thicknesses t1, t2, and t3 satisfy |t1−(t2+t3)|≧1 μm, anda difference between any two values of the thicknesses t1, t2, and t3 isset to 1 μm or more in any case.tα=trα·(tan(θrα)/tan(θo))

In the case where the range of the thickness to of a layer having therefractive index “no” and whose spherical aberration amount falls in apredetermined allowable range is converted into a range of the thicknesstrα of a layer having the refractive index nrα, preferably, thethickness trα may be included in the range of the thickness trα afterconversion.

In the case where the layer between the medium surface and theinformation recording surface or each layer between the informationrecording surfaces is constituted of plural material layers havingrefractive indexes different from each other, at first, the thicknessesof the material layers are calculated in terms of the standardrefractive index. Specifically, the actual thickness of each materiallayer having the refractive index “nr” is converted into the thicknessof each material layer having the standard refractive index “no”, basedon a defocus amount, by multiplying the shape-wise thickness by thefunction value “f”. Then, the thicknesses of the material layers afterconversion are integrated.

For instance, in the case where a cover layer having the shape-wisethickness tr1 has a first cover layer having the refractive index nr11has the thickness tr11, a second cover layer having the refractive indexnr12 has the thickness tr12 . . . , and the N-th cover layer having therefractive index nr1N has the thickness tr1N, converting the shape-wisethickness of the cover layer into the thickness t1 of the cover layerhaving the standard refractive index “no”, based on a defocus amount,yields a result: t1=Σtr1k×f(nrk). In this equation, Σ represents anintegration from 1 through N with respect to “k”.

In the case where an objective lens having a large numerical aperture(NA) is used, spherical aberration sharply changes depending on thethickness of a transparent substrate through which light is transmitted.If the spherical aberration is large, the sensitivity of a focus errorsignal, serving as an index to be used in focus control, may bedifferent from the design sensitivity, or focus error signaldeterioration such as a decrease in signal amplitude may occur.

In the case where focus control is started from a state that focuscontrol is not performed, or stability in focus jumping is obtained, itis desirable to correct spherical aberration with respect to a targetedlayer for focus control in advance. In view of this, it is desirable toset the thickness from the medium surface to an information recordinglayer, and the thickness of an intermediate layer in a predeterminedrange including a standard value.

The focus jumping operation is an operation of changing a focus positionfrom a certain information recording surface to another informationrecording surface. The standard value or a predetermined range for afocus jumping operation should be defined, referring to the sphericalaberration for the above reason. Accordingly, in the case where therefractive index is set to a value other than the standard value, theshape-wise thickness is changed depending on the refractive index.

In view of the above, for instance, the layer thickness of a multilayeroptical disc is designed as follows. First, the refractive index of amaterial constituting a transparent substrate is defined. Next, theshape-wise thickness from the medium surface to an information recordingsurface, and the shape-wise thicknesses of intermediate layers aredetermined in accordance with the obtained refractive index, referringto the spherical aberration. Since it is impossible to set a productionerror to zero, the shape-wise thickness is determined including an errorrange. The shape-wise thickness from the medium surface to aninformation recording surface, and the shape-wise thicknesses ofintermediate layers may be determined, using a numerical value table ora chart. The spherical aberration is proportional to the layerthickness. Accordingly, the shape-wise thickness from the medium surfaceto an information recording surface, and the shape-wise thicknesses ofintermediate layers may be determined by calculating a conversion factorg(n) depending on a refractive index in accordance with a wavelength ora numerical aperture, and using the calculated conversion factor g(n).

For instance, blue light of a wavelength 405 nm is converged on aninformation recording surface through a substrate having a refractiveindex of 1.60 and a thickness of 0.1 mm. An objective lens having anumerical aperture of 0.85 converges blue light of a wavelength 405 nmwithout aberration. The thickness ts(n) (unit: mm) of a substrate whichminimizes the aberration when the refractive index of the substrate ischanged is calculated. As a result of the calculation, the conversionfactor g(n) is set to: g(n)=ts(n)/0.1.

The shape-wise thickness of a cover layer can be obtained, based on theshape-wise thickness from the medium surface to an information recordingsurface, and the shape-wise thicknesses of intermediate layers, whichhave been calculated in the above-described manner. Then, thesethicknesses are converted into thicknesses of the respectivecorresponding layers each having the standard refractive index “no”,referring to a defocus amount in the above-described manner. Then,judgment is made as to whether the back focus problem and the interlayercoherence can be avoided, whether the design range is proper, andwhether the quality of a fabricated optical disc has passed, using thethicknesses of the respective corresponding layers after conversion.

The thickness from the medium surface to an information recordingsurface can be calculated based on the sum of the cover layer thicknessand the intermediate layer thicknesses. In the case of a three-layerdisc, the shape-wise thickness from the medium surface to the firstinformation recording surface is set to tr1, the shape-wise thicknessfrom the medium surface to the second information recording surface isset to (tr1+tr2), and the shape-wise thickness from the medium surfaceto the third information recording surface is set to (tr1+tr2+tr3).

In the case of a four-layer disc, the shape-wise thickness from themedium surface to the first information recording surface is set to tr1,the shape-wise thickness from the medium surface to the secondinformation recording surface is set to (tr1+tr2), the shape-wisethickness from the medium surface to the third information recordingsurface is set to (tr1+tr2+tr3), and the shape-wise thickness from themedium surface to the fourth information recording surface is set to(tr1+tr2+tr3+tr4).

The optical recording medium in the embodiment enables to prevent lightfrom forming an image on the backside of the surface of the opticalrecording medium, and suppress coherence between reflected light fromthe information recording surfaces to thereby improve the quality of aservo signal and a reproduction signal. Further, in the abovearrangement, a guideline for producing the products can be clearly setby setting the guideline for designing the thickness of the opticalrecording medium depending on the refractive index in theabove-described manner.

Next, an example of an optical information device which performs a focusjumping operation is described.

FIG. 12 is a diagram showing a schematic arrangement of an opticalinformation device embodying the invention. Referring to FIG. 12, anoptical information device 150 includes a driver 151, a turntable 152,an electric circuit 153, a clamper 154, a motor 155, and an optical headdevice 201. The optical head device 201 in FIG. 12 has the samearrangement as the arrangement of the optical head device 201 shown inFIG. 1, and an optical recording medium 40 in FIG. 12 has the samearrangement as the arrangement of the optical recording medium 40 shownin FIG. 2.

The optical recording medium 40 is placed on the turntable 152, and isfixedly supported by the clamper 154. The motor 155 rotates theturntable 152 to thereby rotate the optical recording medium 40. Thedriver 151 coarsely drives the optical head device 201 to a track on theoptical recording medium 40 where intended information is recorded.

The optical head device 201 moves the focus position of laser light tobe irradiated onto the optical recording medium from a certaininformation recording surface to another information recording surfaceto reproduce or record information with respect to the pluralinformation recording surfaces.

The optical head device 201 transmits a focus error signal and atracking error signal to the electric circuit 153 in correspondence to apositional relation with respect to the optical recording medium 40. Theelectric circuit 153 transmits a signal for finely moving the objectivelens 56 to the optical head device 201 in accordance with the focuserror signal and the tracking error signal. The optical head device 201performs focus control and tracking control with respect to the opticalrecording medium 40, based on a signal from the electric circuit 153.The optical head device 201 reads out information from the opticalrecording medium 40, writes (records) information into the opticalrecording medium 40, or erases information from the optical recordingmedium 40.

The electric circuit 153 controls and drives the motor 155 and theoptical head device 201, based on a signal to be obtained from theoptical head device 201. The electric circuit 153 mainly controls thefocus jumping sequence. Specifically, the electric circuit 153 controlsthe optical head device 201 in such a manner as to correct sphericalaberration with respect to an information recording surface as a focusjumping destination, before shifting the focus position. A concretespherical aberration correction method for the optical head device 201has been described in the foregoing description.

The optical information device 150 in the embodiment is operable tocorrect spherical aberration with respect to an information recordingsurface as a focus jumping destination by shifting the collimator lens53 with respect to the optical recording medium 40 before a focusjumping operation is performed, and thereafter shift the focus position.This enables to improve the quality of a focus error signal with respectto a targeted information recording surface to thereby stably perform afocus jumping operation.

The optical recording medium embodying the invention is not limited to aspecific one of a read-only disc, a rewritable disc, and a recordabledisc, but may be any of these discs.

The aforementioned embodiment mainly includes the features having thefollowing arrangements.

An optical recording medium according to an aspect of the invention isan optical recording medium having a plurality of information recordingsurfaces. The optical recording medium includes: the first informationrecording surface closest to a medium surface of the optical recordingmedium where light is incident; the second information recording surfacesecond closest to the medium surface; the third information recordingsurface third closest to the medium surface; the fourth informationrecording surface fourth closest to the medium surface; a cover layerhaving a refractive index nr1 and formed between the medium surface andthe first information recording surface; a first intermediate layerhaving a refractive index nr2 and formed between the first informationrecording surface and the second information recording surface; a secondintermediate layer having a refractive index nr3 and formed between thesecond information recording surface and the third information recordingsurface; and a third intermediate layer having a refractive index nr4and formed between the third information recording surface and thefourth information recording surface, wherein in the case whereshape-wise thicknesses tr1, tr2, tr3, and tr4 of the cover layer, thefirst intermediate layer, the second intermediate layer, and the thirdintermediate layer are respectively converted into thicknesses t1, t2,t3, and t4 of the respective corresponding layers each having apredetermined refractive index “no”, a defocus amount with respect to alayer having a refractive index nrα and a thickness trα (satisfying:1≦α≦n (where α is a positive integer and n is an integer of 4 or more)),and a defocus amount with respect to a layer having the refractive index“no” and a thickness tα (satisfying: 1≦α≦n (where α is a positiveinteger and n is an integer of 4 or more)) are equal to each other, andthe thicknesses t1, t2, t3, and t4 satisfy |t1−(t2+t3+t4)|≧1 μm, adifference between any two values of the thicknesses t1, t2, t3, and t4is set to 1 μm or more in any case, and |(t1+t2)−(t3+t4)≧1 μm.

In the above arrangement, in the case where shape-wise thicknesses tr1,tr2, tr3, and tr4 of the cover layer, the first intermediate layer, thesecond intermediate layer, and the third intermediate layer arerespectively converted into thicknesses t1, t2, t3, and t4 of therespective corresponding layers each having a predetermined refractiveindex “no”, a defocus amount with respect to a layer having a refractiveindex nrα and a thickness trα (satisfying: 1≦α≦n (where α is a positiveinteger and n is an integer of 4 or more)), and a defocus amount withrespect to a layer having the refractive index “no” and a thickness tα(satisfying: 1≦α≦n (where α is a positive integer and n is an integer of4 or more)) are equal to each other, and the thicknesses t1, t2, t3, andt4 satisfy |t1−(t2+t3+t4)|≧1 μm, a difference between any two values ofthe thicknesses t1, t2, t3, and t4 is set to 1 μm or more in any case,and |(t1+t2)−(t3+t4)|≧1 μm.

Thus, the thicknesses t1, t2, t3, and t4 obtained by converting theshape-wise thicknesses tr1, tr2, tr3, and tr4 of the cover layer, thefirst intermediate layer, the second intermediate layer, and the thirdintermediate layer satisfy |t1−(t2+t3+t4)|≧1 μm, a difference betweenany two values of the thicknesses t1, t2, t3, and t4 is set to 1 μm ormore in any case, and |(t1+t2)−(t3+t4)|≧1 μm. This enables to preventlight from forming an image on the backside of the surface of theoptical recording medium, and suppress coherence between reflected lightfrom the information recording surfaces to thereby improve the qualityof a servo signal and a reproduction signal. Further, since the distancebetween the surface of the optical recording medium and the informationrecording surface closest to the surface of the optical recording mediumcan be set to a large value, deterioration of a reproduction signal inthe case where there is a damage or a smear on the surface of theoptical recording medium can be suppressed.

An optical recording medium according to another aspect of the inventionis an optical recording medium having a plurality of informationrecording surfaces. The optical recording medium includes: the firstinformation recording surface closest to a medium surface of the opticalrecording medium where light is incident; the second informationrecording surface second closest to the medium surface; the thirdinformation recording surface third closest to the medium surface; thefourth information recording surface fourth closest to the mediumsurface; a cover layer having a refractive index nr1 and formed betweenthe medium surface and the first information recording surface; a firstintermediate layer having a refractive index nr2 and formed between thefirst information recording surface and the second information recordingsurface; a second intermediate layer having a refractive index nr3 andformed between the second information recording surface and the thirdinformation recording surface; and a third intermediate layer having arefractive index nr4 and formed between the third information recordingsurface and the fourth information recording surface, wherein in thecase where shape-wise thicknesses tr1, tr2, tr3, and tr4 of the coverlayer, the first intermediate layer, the second intermediate layer, andthe third intermediate layer are respectively converted into thicknessest1, t2, t3, and t4 of the respective corresponding layers each having apredetermined refractive index “no”, and in the case where a thicknessof the layer having the refractive index nrα is set to trα (satisfying:1≦α≦n (where α is a positive integer and n is an integer of 4 or more)),a convergence angle of light in the layer having the refractive indexnrα is set to θrα (satisfying: 1≦α≦n (where α is a positive integer andn is an integer of 4 or more)), a thickness of the layer having therefractive index “no” is set to tα (satisfying: 1≦α≦n (where α is apositive integer and n is an integer of 4 or more)), and a convergenceangle of light in the layer having the refractive index “no” is set toθo, if the thickness trα is converted into the thickness tα based on thefollowing equation, tα=trα·(tan(θrα)/tan(θo)), the thicknesses t1, t2,t3, and t4 satisfy |t1−(t2+t3+t4)|≧1 μm, a difference between any twovalues of the thicknesses t1, t2, t3, and t4 is set to 1 μm or more inany case, and |(t1+t2)−(t3+t4)|≧1 μm.

In the above arrangement, in the case where shape-wise thicknesses tr1,tr2, tr3, and tr4 of the cover layer, the first intermediate layer, thesecond intermediate layer, and the third intermediate layer arerespectively converted into thicknesses t1, t2, t3, and t4 of therespective corresponding layers each having a predetermined refractiveindex “no”, and in the case where a thickness of the layer having therefractive index nrα is set to trα (satisfying: 1≦α≦n (where a is apositive integer and n is an integer of 4 or more)), a convergence angleof light in the layer having the refractive index nrα is set to θrα(satisfying: 1≦α≦n (where α is a positive integer and n is an integer of4 or more)), a thickness of the layer having the refractive index “no”is set to tα (satisfying: 1≦α≦n (where α is a positive integer and n isan integer of 4 or more)), and a convergence angle of light in the layerhaving the refractive index “no” is set to θo, if the thickness trα isconverted into the thickness tα based on the following equation, thethicknesses t1, t2, t3, and t4 satisfy |t1−(t2+t3+t4)|≧1 μm, adifference between any two values of the thicknesses t1, t2, t3, and t4is set to 1 μm or more in any case, and |(t1+t2)−(t3+t4)|≧1 μm.tα=trα=(tan(θrα)/tan(θo))

Thus, the thicknesses t1, t2, t3, and t4 obtained by converting theshape-wise thicknesses tr1, tr2, tr3, and tr4 of the cover layer, thefirst intermediate layer, the second intermediate layer, and the thirdintermediate layer satisfy |t1−(t2+t3+t4)|≧1 μm, a difference betweenany two values of the thicknesses t1, t2, t3, and t4 is set to 1 μm ormore in any case, and |(t1+t2)−(t3+t4)|≧1 μm. This enables to preventlight from forming an image on the backside of the surface of theoptical recording medium, and suppress coherence between reflected lightfrom the information recording surfaces to thereby improve the qualityof a servo signal and a reproduction signal. Further, since the distancebetween the surface of the optical recording medium and the informationrecording surface closest to the surface of the optical recording mediumcan be set to a large value, deterioration of a reproduction signal inthe case where there is a damage or a smear on the surface of theoptical recording medium can be suppressed.

In the optical recording medium, in the case where a range of thethickness tα of the layer having the refractive index “no” and whosespherical aberration amount falls in a predetermined allowable range isconverted into a range of the thickness trα of the layer having therefractive index nrα, preferably, the thickness trα may be included inthe range of the thickness trα after conversion.

In the above arrangement, in the case where a range of the thickness tαof the layer having the refractive index “no” and whose sphericalaberration amount falls in a predetermined allowable range is convertedinto a range of the thickness trα of the layer having the refractiveindex nrα, the thickness trα is included in the range of the thicknesstrα after conversion. This enables to suppress spherical aberration withrespect to the cover layer, the first intermediate layer, the secondintermediate layer, and the third intermediate layer respectively havingthe thicknesses tr1, tr2, tr3, and tr4.

In the optical recording medium, preferably, the refractive index “no”may be set to 1.60. In this arrangement, the shape-wise thicknesses tr1,tr2, tr3, and tr4 of the cover layer, the first intermediate layer, thesecond intermediate layer, and the third intermediate layer can berespectively converted into the thicknesses t1, t2, t3, and t4 of therespective corresponding layers each having the refractive index of1.60.

In the optical recording medium, preferably, the thicknesses t1, t2, t3,and t4 may each be set to 10 μm or more. In this arrangement, settingthe thicknesses t1, t2, t3, and t4 each to 10 μm or more enables toreduce an influence of crosstalk from an adjacent information recordingsurface to thereby reduce coherence between reflected light from theinformation recording surfaces.

An optical recording medium according to another aspect of the inventionis an optical recording medium having a plurality of informationrecording surfaces. The optical recording medium includes: the firstinformation recording surface closest to a medium surface of the opticalrecording medium where light is incident; the second informationrecording surface second closest to the medium surface; the thirdinformation recording surface third closest to the medium surface; acover layer having a refractive index nr1 and formed between the mediumsurface and the first information recording surface; a firstintermediate layer having a refractive index nr2 and formed between thefirst information recording surface and the second information recordingsurface; and a second intermediate layer having a refractive index nr3and formed between the second information recording surface and thethird information recording surface, wherein in the case whereshape-wise thicknesses tr1, tr2, tr3, and tr4 of the cover layer, thefirst intermediate layer, and the second intermediate layer arerespectively converted into thicknesses t1, t2, t3, and t4 of therespective corresponding layers each having a predetermined refractiveindex “no”, a defocus amount with respect to a layer having a refractiveindex nrα and a thickness trα (satisfying: 1≦α≦n (where α is a positiveinteger and n is an integer of 3 or more)), and a defocus amount withrespect to a layer having the refractive index “no” and a thickness tα(satisfying: 1≦α≦n (where α is a positive integer and n is an integer of3 or more)) are equal to each other, and the thicknesses t1, t2, and t3satisfy |t1−(t2+t3)|≧1 μm, and a difference between any two values ofthe thicknesses t1, t2, and t3 is set to 1 μm or more in any case.

In the above arrangement, in the case where shape-wise thicknesses tr1,tr2, tr3, and tr4 of the cover layer, the first intermediate layer, andthe second intermediate layer are respectively converted intothicknesses t1, t2, t3, and t4 of the respective corresponding layerseach having a predetermined refractive index “no”, a defocus amount withrespect to a layer having a refractive index nrα and a thickness trα(satisfying: 1≦α≦n (where α is a positive integer and n is an integer of3 or more)), and a defocus amount with respect to a layer having therefractive index “no” and a thickness tα (satisfying: 1≦α≦n (where α isa positive integer and n is an integer of 3 or more)) are equal to eachother, and the thicknesses t1, t2, and t3 satisfy |t1−(t2+t3)|≧1 μm, anda difference between any two values of the thicknesses t1, t2, and t3 isset to 1 μm or more in any case.

Thus, the thicknesses t1, t2, and t3 obtained by converting theshape-wise thicknesses tr1, tr2, and tr3 of the cover layer, the firstintermediate layer, and the second intermediate layer satisfy|t1−(t2+t3)|≧1 μm, and a difference between any two values of thethicknesses t1, t2, and t3 is set to 1 μm or more in any case. Thisenables to prevent light from forming an image on the backside of thesurface of the optical recording medium, and suppress coherence betweenreflected light from the information recording surfaces to therebyimprove the quality of a servo signal and a reproduction signal.Further, since the distance between the surface of the optical recordingmedium and the information recording surface closest to the surface ofthe optical recording medium can be set to a large value, deteriorationof a reproduction signal in the case where there is a damage or a smearon the surface of the optical recording medium can be suppressed.

An optical recording medium according to another aspect of the inventionis an optical recording medium having a plurality of informationrecording surfaces. The optical recording medium includes: the firstinformation recording surface closest to a medium surface of the opticalrecording medium where light is incident; the second informationrecording surface second closest to the medium surface; the thirdinformation recording surface third closest to the medium surface; acover layer having a refractive index nr1 and formed between the mediumsurface and the first information recording surface; a firstintermediate layer having a refractive index nr2 and formed between thefirst information recording surface and the second information recordingsurface; and a second intermediate layer having a refractive index nr3and formed between the second information recording surface and thethird information recording surface, wherein in the case whereshape-wise thicknesses tr1, tr2, and tr3 of the cover layer, the firstintermediate layer, and the second intermediate layer are respectivelyconverted into thicknesses t1, t2, and t3 of the respectivecorresponding layers each having a predetermined refractive index “no”,and in the case where a thickness of the layer having the refractiveindex nrα is set to trα (satisfying: 1≦α≦n (where α is a positiveinteger and n is an integer of 3 or more)), a convergence angle of lightin the layer having the refractive index nrα is set to θrα (satisfying:1≦α≦n (where α is a positive integer and n is an integer of 3 or more)),a thickness of the layer having the refractive index “no” is set to tα(satisfying: 1≦α≦n (where α is a positive integer and n is an integer of3 or more)), and a convergence angle of light in the layer having therefractive index “no” is set to θo, if the thickness trα is convertedinto the thickness tα based on the following equation,tα=trα(tan(θrα)/tan(θo)), the thicknesses t1, t2, and t3 satisfy|t1−(t2+t3)|≧1 μm, and a difference between any two values of thethicknesses t1, t2, and t3 is set to 1 μm or more in any case.

In the above arrangement, in the case where shape-wise thicknesses tr1,tr2, and tr3 of the cover layer, the first intermediate layer, and thesecond intermediate layer are respectively converted into thicknessest1, t2, and t3 of the respective corresponding layers each having apredetermined refractive index “no”, and in the case where a thicknessof the layer having the refractive index nrα is set to trα (satisfying:1≦α≦n (where α is a positive integer and n is an integer of 3 or more)),a convergence angle of light in the layer having the refractive indexnrα is set to θrα (satisfying: 1≦α≦n (where α is a positive integer andn is an integer of 3 or more)), a thickness of the layer having therefractive index “no” is set to tα (satisfying: 1≦α≦n (where α is apositive integer and n is an integer of 3 or more)), and a convergenceangle of light in the layer having the refractive index “no” is set toθo, if the thickness trα is converted into the thickness tα based on thefollowing equation, the thicknesses t1, t2, and t3 satisfy|t1−(t2+t3)|≧1 μm, and a difference between any two values of thethicknesses t1, t2, and t3 is set to 1 μm or more in any case.tα=trα·(tan(θrα)/tan(θo))

Thus, the thicknesses t1, t2, and t3 obtained by converting theshape-wise thicknesses tr1, tr2, and tr3 of the cover layer, the firstintermediate layer, and the third intermediate layer satisfy|t1−(t2+t3)|≧1 μm, and a difference between any two values of thethicknesses t1, t2, and t3 is set to 1 μm or more in any case. Thisenables to prevent light from forming an image on the backside of thesurface of the optical recording medium, and suppress coherence betweenreflected light from the information recording surfaces to therebyimprove the quality of a servo signal and a reproduction signal.Further, since the distance between the surface of the optical recordingmedium and the information recording surface closest to the surface ofthe optical recording medium can be set to a large value, deteriorationof a reproduction signal in the case where there is a damage or a smearon the surface of the optical recording medium can be suppressed.

In the optical recording medium, in the case where a range of thethickness tα of the layer having the refractive index “no” and whosespherical aberration amount falls in a predetermined allowable range isconverted into a range of the thickness trα of the layer having therefractive index nrα, preferably, the thickness trα may be included inthe range of the thickness trα after conversion.

In the above arrangement, in the case where a range of the thickness tαof the layer having the refractive index “no” and whose sphericalaberration amount falls in a predetermined allowable range is convertedinto a range of the thickness trα of the layer having the refractiveindex nrα, the thickness trα is included in the range of the thicknesstrα after conversion. This enables to suppress spherical aberration withrespect to the cover layer, the first intermediate layer, and the secondintermediate layer respectively having the thicknesses tr1, tr2, andtr3.

In the optical recording medium, preferably, the refractive index “no”may be set to 1.60. In this arrangement, the shape-wise thicknesses tr1,tr2, and tr3 of the cover layer, the first intermediate layer, and thesecond intermediate layer can be respectively converted into thethicknesses t1, t2, and t3 of the respective corresponding layers eachhaving the refractive index of 1.60.

In the optical recording medium, preferably, the thicknesses t1, t2, andt3 may each be set to 10 μm or more. In this arrangement, setting thethicknesses t1, t2, and t3 each to 10 μm or more enables to reduce aninfluence of crosstalk from an adjacent information recording surface tothereby reduce coherence between reflected light from the informationrecording surfaces.

An optical information device according to another aspect of theinvention is an optical information device for reproducing or recordingwith respect to the optical recording medium having any one of the abovearrangements. The optical information device includes an optical headdevice, and a motor which rotates the optical recording medium. Theoptical head device shifts a focus position of the laser light to beirradiated onto the optical recording medium from a certain informationrecording surface to another information recording surface out of theplurality of the information recording surfaces to reproduce or recordinformation with respect to the plurality of the information recordingsurfaces. In this arrangement, the above optical recording medium can beused in the optical information device.

Preferably, the optical information device may further include anelectric circuit which controls and drives the motor and the opticalhead device, based on a signal to be obtained from the optical headdevice, wherein the electric circuit controls the optical head device insuch a manner as to correct a spherical aberration with respect to atargeted information recording surface out of the plurality of theinformation recording surfaces, before shifting the focus position.

According to the above arrangement, spherical aberration with respect toa targeted information recording surface can be corrected before thefocus position is shifted.

The embodiments or the examples described in the description ofembodiments are provided to clarify the technical contents of theinvention. The invention should not be construed to be limited to theembodiments or the examples. The invention may be modified in variousways as far as such modifications do not depart from the spirit and thescope of the invention hereinafter defined.

INDUSTRIAL APPLICABILITY

The inventive multilayer optical disc (the inventive optical recordingmedium) and the inventive optical information device enable to maximallysuppress an influence of reflected light from an information recordingsurface other than a targeted information recording surface at the timeof reproducing from the targeted information recording surface, even ifthe refractive indexes of the cover layer and the intermediate layer aredifferent from the standard value, to thereby reduce an influence to aservo signal and a reproduction signal to be used in an optical headdevice. Thus, the invention is useful to an optical recording medium forinformation recording or reproducing by irradiated light, and an opticalinformation device which records or reproduces information with respectto the optical recording medium.

Thus, the invention provides an optical recording medium capable ofsecuring a reproduction signal of good quality, having a large capacity,and having compatibility with an existing optical recording medium.

1. An optical recording medium having a plurality of informationrecording surfaces, the optical recording medium comprising: a firstinformation recording surface closest to a medium surface of the opticalrecording medium where light is incident; a second information recordingsurface second closest to the medium surface; a third informationrecording surface third closest to the medium surface; a cover layerhaving a refractive index nr1 and formed between the medium surface andthe first information recording surface; a first intermediate layerhaving a refractive index nr2 and formed between the first informationrecording surface and the second information recording surface; and asecond intermediate layer having a refractive index nr3 and formedbetween the second information recording surface and the thirdinformation recording surface, wherein in a case where shape-wisethicknesses tr1, tr2, and tr3 of the cover layer, the first intermediatelayer, and the second intermediate layer are respectively converted intothicknesses t1, t2, and t3 of the respective corresponding layers eachhaving a predetermined refractive index “no”, and in a case where athickness of a layer having a refractive index nrα is set to trα(satisfying: 1≦α≦n (where α is a positive integer and n is an integer of3 or more)), a convergence angle of light in the layer having therefractive index nrα is set to θrα (satisfying: 1≦α≦n (where α is apositive integer and n is an integer of 3 or more)), a thickness of thelayer having the refractive index “no” is set to tα (satisfying: 1≦α≦n(where α is a positive integer and n is an integer of 3 or more)), and aconvergence angle of light in the layer having the refractive index “no”is set to θo, if the thickness trα is converted into the thickness tαbased on the following equation,tα=trα(tan(θrα)/tan(θo)), the thicknesses t1, t2, and t3 satisfy|t1−(t2+t3)≧1 μm, and a difference between any two values of thethicknesses t1, t2, and t3 is set to 1 μm or more in any case.
 2. Anoptical information device for reproducing or recording with respect tothe optical recording medium of claim 1, the optical information devicecomprising: an optical head device; and a motor which rotates theoptical recording medium, wherein the optical head device shifts a focusposition of a laser light to be irradiated onto the optical recordingmedium from a certain information recording surface to anotherinformation recording surface out of the plurality of the informationrecording surfaces to reproduce or record information with respect tothe plurality of the information recording surfaces.